Downloaded from http://egsp.lyellcollection.org/ at University of on March 9, 2016

Coastal erosion and slope instability at Downderry, south-east : an outline of the problem and its implications for planning

M. A. Coard, P. C. Sims & J. L. Ternan

ABSTRACT: The Quaternary head deposits which overlay a wave-cut shore platform along a 2 km sector of coast between Seaton and Downderry in south-east Cornwall, have been subjected to significant coastal erosion over many years. As a result, a number of landslips have occurred with considerable damage and even loss of cliff top dwellings. An outline of recent investigations conducted on the cliff and landslip sections with regard to slope stability and the nature of the head deposits is presented. Attention is drawn to the need for adequate drainage of the cliff material, particularly in the siting of gutter soakaways from the cliff top settlements. Despite the history of landslipping in this coastal sector, planning permissions have been granted and a number of new dwellings have been constructed on the cliffs. Ultimately, these properties will be affected by the receding cliff edge and associated slope failures. The paper thus reviews the efficacy of both public and private coastal defence schemes, particularly in relation to the overall stability of the head materials, beach sediment budgets and the 'natural' protection provided by longshore sediment transport at the foot of the cliffs.

Introduction across the exposed coastal areas during a number of the Pleistocene 'cold phases' when sea level was lower. These Man's predilection to use the coastal zone for residential glacio-eustatic changes culminated in a sea level rise purposes frequently highlights misunderstanding of the during the post-glacial (Flandrian) period. dynamic nature of such environments and ignorance of In the coastal sector between Seaton and Downderry, the processes responsible for coastal erosion. A number the local bedrock of lower Devonian slates, has provided of well known cases exist of active coastal recession which the parent materials to establish an extensive solifluction threaten adjacent properties or even substantial settle- terrace burying a low level rock platform at 0-2 m OD. ment, as, for example, at Barton-on-Sea (Barton & Coles This wave-cut feature, dating originally from the late 1984). The 2 km coastal sector between Seaton and Pleistocene (Devensian), emerges at low tide from Downderry in south-east Cornwall (Fig. 1) has not beneath the modern beach sediments and is currently previously received attention in the literature, although being actively trimmed. The soliflucted materials form a the threat to properties and the coastal road is probably flat-topped feature nearly 2 km in length, varying in width equal to other better known examples. The village of from less than 5 m at Seaton to 130 m east of Downderry Downderry is sited on a low cliff of Pleistocene soliflucted village. The thickness of the terrace ranges from 3-4 m in materials, and property development has continued the east of the section to a substantial cliff some 25 m in throughout the present century despite the obvious height near Seaton. Inland, the low rock platform (here continuation of cliff undercutting and erosion. forming a fossil cliff) emerges from beneath the younger deposits landward of the many cliff-top houses that have been constructed, during the present century, along the Background B3247 coast road (Fig. 2). The nature of the head deposits of the solifluction In order to fully explain the nature of the cliff materials terrace, characterised by unconsolidated, poorly sorted and their subsequent erosion, it is necessary to present and angular slate fragments in a silt and clay matrix with some detail of the geomorphological history of these numerous down-slope bedding planes, renders the head types of coastal deposit. Throughout south-west Eng- of little resistance to erosion by waves or surface running land, sea level changes resulting from glacio-eustasy have water. Furthermore, landsliding is aided by saturation of produced coastlines characterised by wave-cut rock plat- the head materials above less permeable silt and clay forms, some of which are frost-shattered or covered by horizons. The undercutting of the base of the cliff by variable thicknesses of locally derived solifluction depo- winter storms has led many cliff-top property owners to sits. These sediments (known generally as 'head') spread protect the base of the cliffs with rubble, concrete,

From CULSHAW,M. G., BELL, F. G., CRIPPS, J. C. & O'HARA, M. (eds) 1987. Planning and Engineering Geology, Geological Society Engineering Geology Special Publication No. 4, pp. 529-32. Downloaded from http://egsp.lyellcollection.org/ at University of Plymouth on March 9, 2016

530 M.A. COARD, P. C. SIMS & J. L. TERNAN

N

83247

Solifluction terrace !Z'!~:! 5:~:! S I a t e s :.'i!~:~.:i':!.::!:i~-iii:i :., [ Head "" -.-. ~ ? ~ "~, Potential rotational slip plane

::i!ii~i':iiiii-!ii!:i~iZ~.'!!'i::: ::i: ~-!.:~:.i~~:.;!i'::i=.::i:i~'!iif!:::!~ " :: :)' .. :.: !::: .;::i;' :'fi.: :i i;i~()...... Sea ~' :~::?)':s !.:..: :::::( .:.; :':).!:wave cut .):.i, :~... ::' :

I kilometres 15 0 mi'les 10 FIG. 2. Schematic diagram of the solifluction terrace at Down- FIG. 1. The coast of south-east Cornwall. derry.

masonry walling or stone-filled gabions. It is evident that to such a degree that the building was no longer habitable. the erosion problem has existed for a number of years and Rotational slumps exposed other house foundations and it can be attributed to both long-term and more recent resulted in a 20 m downthrow of material towards the events. beach. Further cliff falls occurred subsequently, and the The deposits have suffered continued erosion from two local District Council's engineers suggested that exten- main causes. Firstly, as a result of the progressive rise in sions to the cliff top dwellings had resulted in excessive sea levels over the last 15 000 years, which according to loading, leading to instability. Thomas (1985) is still occurring at a rate of 2 mm per annum in south-west . Secondly, superimposed The position of roof water soakaways is also of interest. upon this are the influences of surges and modern storm The storms of February 1974, whose wave action under- wave activity. These processes have resulted in significant cut the base of the cliffwere associated with simultaneous losses of land. heavy rain. Over 200 mm ( 8 inches) of rain fell in the Evidence derived from Ordnance Survey maps at month, most of which came in five precipitation events of 1 : 1250 and 1 : 10560 scales in 1880, 1907, 1952 and 1964 over 25 mm (1 inch). Rainfall intensity averaged 3 mm per indicates that the width of the beach (that is the measured hour for all of these events with ranges from 1.6 to 5.3 mm distance between MHWOT and MLWOT) between per hour. Thus, large quantities of water were directed Seaton and Downderry has been reduced by some from house roofs, via downpipes, to garden soakaways 50m--equivalent to a rate of 0.7 m per year. This sited only a few metres from the property. The combina- removal of beach sediment could be linked to continued tion of rapid saturation of the downslope bedded solif- slow sea level rise (as indicated above) but is more likely a lucted materials, together with erosion at the foot of the response to changes in the nearshore circulation of tow cliff led to major slipping along rotational shear sediments and longshore drifting. Certainly, the con- planes (Fig. 2). struction of the small sea wall to the west of the Seaton Since 1974, further erosion of the solifluction terrace river and the major coastal protection works further west has occurred at the seaward face. The cliff regularly spalls at have cut off some local sediment supply pre- or slumps onto the beach and from time to time small viously brought to Seaton/Downderry by an easterly mudflows can be observed, although evidence is rapidly longshore drift. This loss of material has resulted in a removed by subsequent wave action at the next high tide. reduction in natural beach protection and virtually every Such is the pace of retreat that in 1981 the coast road was storm now has the ability to attack the base of the threatened at the western end of the sector. Cracks of l0 solifluction terrace. mm width appeared in the road surface which caused the Concern by local residents about sea defence resulted in County Council (as highway authority) to draw up plans a public inquiry in 1959. Removal of sediment at Seaton for protection of this part of the cliff and a new sea wall beach (sand and gravel abstraction) was seen as a was completed in 1983. However, whilst protection is contributory cause but the investigation concluded that a afforded to the road, the wall ends at the site of the 1974 s scheme for coastal protection works was 'unjusti- slip (Figs 3 and 4) and thus properties here, and further fied'. Since 1959, there has been significant erosion despite east, still require protection. Consulting engineers the cessation of sediment removal. retained by the local District Council consider that a In February 1974, two houses at the western end of the number of properties in the immediate area are poten- terrace lost their gardens due to landslipping. The tially at risk and estimated (1985) the cost of building a foundations of one of these properties were undermined 155 m sea wall to adjoin the County Council's scheme to Downloaded from http://egsp.lyellcollection.org/ at University of Plymouth on March 9, 2016

COASTAL EROSION AND SLOPE INSTABILITY, SE CORNWALL 531

FIG. 3. Site of the 1974 rotational slip showing the end of the County Council new sea wall (completed 1983). FIG. 5. Failed private sea wall with a vegetated (stabilized) slope behind and the site of the new bungalows with no sea wall protection.

despite the ultimate failure of the private sea defence schemes, a noticeable retardation in the rate of cliff erosion has resulted. The prevention of continuous removal and slumping has allowed the establishment of an extensive vegetation cover on the cliff face, which, in turn, imparts an increased stability to the cliff materials (Fig. 5). The longevity and degree of protection afforded by these privately funded 'do-it-yourself' structures var- ies considerably, but their effectiveness can be quite significant.

Nature of cliff materials

Fro. 4. Comparison between the regraded slope behind the new Over the last five years, work by the authors has been sea wall and the 1974 landslip zone. undertaken to establish the nature and properties of the cliff materials in conjunction with studies of current erosion, slumping and subsequent cliff recession. be s a price well in excess of the market value of Although no major slumping, of a comparable scale to the properties protected. Thus, the extension of the sea the 1974 events, has taken place over the last 12 years, wall is unlikely to attract any government grant aid or minor cliff falls regularly occur and one resident reports even local funding. up to 10 m loss of hisland since that time. It is interesting to note that a number of residents, The materials which make up the matrix of the head whose properties are at risk, have, over a number of years, can be divided into coarser and finer components. resorted to the construction of 'do-it-yourself' sea Throughout, these materials are poorly sorted, and defences ranging from major concrete structures to partial stratification gives rise to distinct lenses, or layers, relatively cheap stone-filled gabions. In certain cases, in many parts of the cliff section.

TABLE 1. Properties of materials.

Head matrix ( < 2 mm fraction) %sand %salt %clay Coarser layers 75-95 5-18 5-10 Finer layers 40 50 10 A tterberg limits: Plastic limit = 14-19% Liquid limit (cone penetrometer technique)= 25-35~ Direct shear test (remoulded < 2 mm material): Cohesion (c') =0-O.0 5 kg cm -2 Angle of shearing resistance (05')= 31-33 ~ Downloaded from http://egsp.lyellcollection.org/ at University of Plymouth on March 9, 2016

532 M. A. COARD, P. C. SIMS & J. L. TERNAN

The percentage of sand-sized materials is very high Deposite knowledge of the major slump of 1974, the (Table 1) and it is notable that, even in the finer horizons, subsequent inquiry, the construction of a sea wall by the the clay content rarely exceeds 10 per cent. The sand County Council (Figs 3 and 4) and concern by the fraction consists of small fragments of slate and the Environmental Services Department of the local District further breakdown of the slate accounts for the silt-sized Council, it seems astonishing that planning permissions material. Of particular importance to slope stability is the have been granted for the building of new properties in occurrence of a finer horizon near the base of the head close proximity to the receding cliffedge. Fig. 5 illustrates succession. This layer is characterised by an absence of the nature of the current situation. To the right of the coarse (> 2 mm) fragments, is very compact and of low terrace of older houses and on top of the solifluction permeability. In the late winter and spring, a distinct materials (low cliff) can be seen the roofs of two new seepage zone occurs near the boundary of this horizon bungalows which have been built in the last three years. and the overlying coarser units of the head. The build up Below this site, at the base of the cliff, there is no form of of water above this horizon during the wetter weather protection from regular wave attack, let alone storm wave increases pore water pressures in the head materials. This, activity. It is likely that the market value of these new together with more active marine undercutting, may properties will decline as cliff erosion proceeds, culminat- account for the generally greater frequency of slope ing in major structural damage within the normal life failure in the late winter and early spring. expectancy of these buildings. What is particularly impor- tant, in view of the 1974 failure, is that adequate drainage is provided for all dwellings along the cliff top. Such drainage should ensure that the concentrated discharge of intercepted surface water is not directed into soakaways Discussion which cause local rapid saturation of the head materials and thus increase the likelihood of rotational slumping. The work undertaken in reviewing both the history of erosion in this area and the nature and properties of the cliffmaterials suggests that active marine erosion, primar- ily generated by storm wave activity, is the main cause of Conclusion coastal recession. Despite localized (private) attempts at sea wall construction, erosion continues due to the The nature of the coast in this area of south-east Cornwall piecemeal nat-ure of such undertakings. Owing to the has been outlined together with a review of the history of local erosion problems. Recent studies undertaken by the discontinuous nature of these defences, wave attack works around the side and, eventually, behind these authors indicate the likelihood of continued marine structures, which ultimately collapse (Fig. 5). In some erosion resulting in cliff failures along this coastal sector. Natural protection afforded by longshore supplies of cases, the lack of adequate drainage through concrete or sediment seems to have diminished, exacerbating the masonry walling has led to a build up of pore water problem. To this basic framework is added the complica- pressures on the landward side, leading to major fractures tion of cliff-top settlements and the exact role of county and failure of the structure. Quite often, inadequate toe and local authorities in coastal protection. In the absence protection provided by deeper foundations or vertical piling leads to these simple sea walls just 'toppling over' of a general coastal zone management plan for this area, when even moderate wave action has shifted or removed apparently contradictory decisions have been made the narrow zone of beach sediments which offer only resulting in extensions to existing dwellings or even the construction of new properties. In the context of coastal short term natural protection to such low cost schemes. To stop erosion completely would require the conti- erosion and protection there would appear, therefore, to nuation of the County Council's sea wall along the whole be a need for closer liaison between departments within length of the coast at Downderry. Such remedial local authorities, and between such authorities and measures are out of the question, in relation to the total scientists and engineers undertaking coastal research. In value of the properties under immediate threat, due to the this way, both public and private interests can be high costs of such undertakings. Thus, erosion is likely to safeguarded. continue and it is difficult to predict at what point recession will cease, especially as the existing exposed wave cut platform, together with the buried platform, are References at modern sea level and thus cannot provide any signifi- BARTON, M. E. & COLES, B. J. 1984. The characteristics and rates cant rock rampart for the dissipation of incoming wave of the various slope degradation processes in the Barton Clay energy. In fact, the gentle slope of the platform is more cliffs of Hampshire. Quarterly Journal of Engineering Geology likely to create steeper wave fronts as approaching 17, 117-36. breakers encounter frictional retardation in the shallow- THOMAS, C. 1985. Exploration of a drowned landscape." Archae- ing water. ology and history of the Isles of Scilly. Batsford, London.

M. A. COARD, P. C. SIMS• J. L. TERNAN, Department of Geographical Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA, UK